Aluminum die-cast alloys have high dimensional accuracy and smooth and attractive casting surfaces in the as-cast condition. In addition, they can be produced in large quantities in a rapid operation. Therefore, aluminum die-cast alloys are extensively used as daily necessities, machine parts, etc.
Conventional aluminum die-casting alloys are based on Al-Si (JIS-ADC1), Al-Si-Cu (ADC10, ADC12), Al-Si-Mg (ADC3) and Al-Mg (ADC5) systems and aluminum alloys containing 7.5-13 wt % Si are used most commonly in die-casting (The term "JIS" as used herein refers to a "Japanese Industrial Standard"). The alloying of aluminum with silicon provides various advantages such as improving fluidity, reducing solidification shrinkage, decreasing the thermal expansion coefficient, and improving wear resistance. However, aluminum alloyed with silicon is very low in elongation and impact resistance (toughness) because silicon is brittle and the silicon in the eutectic crystals of Al-Si grow in the form of long needles.
As modern engineering products have gained ever higher performance levels and as their ranges of application have expanded, an increasing number of customers have required die-cast products having improved mechanical strength and toughness, and these requirements cannot be fully met by the usual practice of adjusting the composition of JIS alloys or incorporating certain technical improvements in the casting process. Protective automotive parts are required to have high toughness and, conventionally their specified quality is attained by performing a T.sub.6 tempering (heat treatment) on castings of Al-Si-Mg and other AC4C alloys (aluminum casting alloys) specified in JIS having comparatively high toughness. However, performing heat treatments after casting is highly inefficient and undesirably increases product costs. Conventionally used Al-Si-Cu alloys do not have high toughness, since tabular eutectic Si crystals form upon solidification, and these alloys are not suitable for use in protective parts which are required to have high toughness. Under these circumstances, it is desired to develop casting alloys that exhibit high strength and toughness in the as-cast condition.
The aluminum alloys of ADC5 and ADC6 type are widely used in castings where high corrosion resistance is required and as alloys for making Alumite having oxidation protecting film formed by anodic oxidation. Binary Al-Mg alloys have high corrosion resistance comparable to that of engineering pure aluminum but they experience seizure in or welding to dies extensively and, because of alloying with magnesium, the range of temperatures at which these alloys solidify is expanded to cause cracking and reduced hot fluidity. In practical applications, up to 1% Si and trace amounts of Mn and Fe are added to ADC6 so as to improve its castability and strength.
As for ADC,5, seizure in dies is inhibited by addition of up to 1.8% Fe, so that this alloy can be die-cast.
As described above, in order to improve castability and strength without sacrificing corrosion resistance, commercial Al-Mg base die-casting alloys contain comparatively small amounts of elements such as Si, Fe and Mn, either independently or in combination. However, these alloys are principally intended to be used in applications where high corrosion resistance is required and their tensile strength, yield point and modulus are generally low as compared with ADC10 and ADC12. Therefore, these alloys, which can be used in ornamental parts such as cases and covers, find limited use as structural materials.
Aluminum alloys containing 7.5-13 wt % Si such as JIS ADC1 (Al-Si type), ADC10, ADC12 and ADC3 (Al-Si-Mg type) are sometimes anodized and used in sliding members such as pistons and cylinder liners. However, because silicon is a strong current retarding element, the formation of a desired anodic oxide film will not proceed unless desiliconization is effected, such as by pickling, or anodization is performed with specific waveforms of electric current. It is therefore a very difficult task which requires complicated procedures to provide Si-containing aluminum alloys with anodic films that are 20-30 .mu.m thick, in which range they exhibit particularly high wear resistance.